Production and use of tetrafluorosilane

ABSTRACT

Tetrafluorosilane is produced by a process comprising a step (1) of heating a hexafluorosilicate, a step (2-1) of reacting a tetrafluorosilane gas containing hexafluorodisiloxane produced in the step (1) with a fluorine gas, a step (2-2) of reacting a tetrafluorosilane gas containing hexafluorodisiloxane produced in the step (1) with a highvalent metal fluoxide, or a step (2-1) of reacting a tetrafluorosilane gas containing hexafluorodisiloxane produced in the step (1) with a fluorine gas and a step (2-3) of reacting a tetrafluorosilane gas produced in the step (2-1) with a highvalent metal fluoride. Further, impurities in high-purity tetrafluorosilane are analyzed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofthe Provisional Application 60/306,420 filed Jul. 20, 2001, pursuant to35 U.S.C. §111(b).

TECHNICAL FIELD

[0002] The present invention relates to a production process oftetrafluorosilane, a method for analyzing impurities in a high-puritytetrafluorosilane, and uses thereof.

BACKGROUND ART

[0003] Tetrafluorosilane (hereinafter sometimes referred to as “SiF₄”)is used, for example, as a raw material for optical fibers,semiconductors or solar cells, and a high-purity product is required. Asfor the production process thereof, for example, a method of producingSiF₄ by reacting SiO₂ and HF in the presence of concentrated sulfuricacid is known (Japanese Unexamined Patent Publication No. 57-135711(JP-A-57-135711)).

[0004] However, this method has a problem in that water is produced as aby-product upon reaction of the raw materials SiO₂ and HF. The waterproduced may be removed by a concentrated sulfuric acid but cannot becompletely removed and the SiF₄ produced disadvantageously contains alarge amount of HF and hexafluorodisiloxane ((SiF₃)₂O) produced by thereaction of water and SiF₄ and, further, contains carbon dioxide whichis difficult to separate from SiF₄ and which is considered to originatefrom the slight amount of a carbon compound contained in theconcentrated sulfuric acid.

[0005] Also, a method for producing SiF₄ by thermally decomposing ahexafluorosilicate is known. However, the hexafluorosilicate containsH₂O or impurities such as trace oxygen-containing silicic acid compounds(e.g., SiO₂) and unless pretreated satisfactorily, the impurity mayreact with SiF₄ to produce hexafluorodisiloxane when the thermaldecomposition is performed.

[0006] A method for purifying SiF₄ containing (SiF₃)₂O, CO₂ or HF isalso known. In the case where SiF₄ contains impurity gases such as(SiF₃)₂O, CO₂ and O₂, it is known that if the SiF₄ is used, for example,as a raw material of a silicon thin film, a mixing of oxygen is causedand this adversely affects the characteristics of a semiconductor orfiber. Accordingly, high-purity SiF₄ reduced in impurities is requiredand, as one of the evaluation techniques, an analysis method for traceimpurities is also required.

[0007] With respect to the method for purifying SiF₄, for example,Japanese Unexamined Patent Publication No. 57-156317 (JP-A-57-156317)describes a method for purifying SiF₄ containing (SiF₃)₂O by contactingit with an adsorbent. However, when the adsorbent used in this method isregenerated and used, the initial adsorbing capability is not broughtout in some cases. The reason therefor is not clearly known but it isconsidered to be because adsorbed hexafluorodisiloxane is decomposedwithin pores of the adsorbent. As SiO₂ produced by the decompositionattaches to the adsorption site, the adsorbent cannot be regenerated andre-used and this causes a problem that the adsorbent must be treated asa waste. Furthermore, if the adsorbent is not sufficiently baked beforethe passing of gas, a side reaction with water content takes place toproduce hexafluorodisiloxane instead.

DISCLOSURE OF INVENTION

[0008] The present invention has been made under these circumstances andthe object of the present invention is to provide a production processof tetrafluorosilane, a method for analyzing impurities in a high-puritytetrafluorosilane, and uses thereof.

[0009] As a result of extensive investigations to solve theabove-described problems, the present inventors have found that theseproblems can be solved by using a process, for producingtetrafluorosilane, comprising a step (1) of heating ahexafluorosilicate, a step (2-1) of reacting a tetrafluorosilane gascontaining hexafluorodisiloxane produced in the step (1) with a fluorinegas, a step (2-2) of reacting a tetrafluorosilane gas containinghexafluorodisiloxane produced in the step (1) with a highvalent metalfluoxide, or a step (2-1) of reacting a tetrafluorosilane gas containinghexafluorodisiloxane produced in the step (1) with a fluorine gas and astep (2-3) of reacting a tetrafluorosilane gas produced in the step(2-1) with a highvalent metal fluoxide.

[0010] The present inventors have also found that those problems can besolved by using a method for analyzing impurities in a high-puritytetrafluorosilane, comprising bringing tetrafluorosilane containing H₂gas, O₂ gas, N₂ gas, CO gas, CH₄ gas and/or CO₂ gas as impurities intocontact with an adsorbent to separate the impurities fromtetrafluorosilane, and introducing the impurities together with acarrier gas into a gas chromatograph to analyze the impurities.

[0011] The present invention has been accomplished based on thesefindings.

[0012] Therefore, the present invention provides a process, forproducing tetrafluorosilane, comprising a step (1) of heating ahexafluorosilicate, a step (2-1) of reacting a tetrafluorosilane gascontaining hexafluorodisiloxane produced in the step (1) with a fluorinegas, a step (2-2) of reacting a tetrafluorosilane gas containinghexafluorodisiloxane produced in the step (1) with a highvalent metalfluoxide, or a step (2-1) of reacting a tetrafluorosilane gas containinghexafluorodisiloxane produced in the step (1) with a fluorine gas and astep (2-3) of reacting a tetrafluorosilane gas produced in the step(2-1) with a highvalent metal fluoxide.

[0013] The present invention also provides a high-puritytetrafluorosilane having a hexafluorodisiloxane content of 1 vol ppm orless, which is obtained by the production process described above.

[0014] The present invention also provides a method for analyzingimpurities in a high-purity tetrafluorosilane, comprising bringing atetrafluorosilane gas containing an H₂ gas, an O₂ gas, an N₂ gas, a COgas, a CH₄ gas and/or a CO₂ gas as impurities into contact with anadsorbent to separate the impurities from tetrafluorosilane, andintroducing the impurities together with a carrier gas into a gaschromatograph to analyze the impurities.

[0015] The present invention also provides a method for analyzingimpurities in a high-purity tetrafluorosilane, comprising introducing atetrafluorosilane gas containing hexafluorodisiloxane as an impurityinto a cell with the material of window being composed of a metalhalide, and analyzing the hexafluorodisiloxane and/or hydrogen fluorideby infrared spectrometry.

[0016] The present invention further provides a gas for the productionof an optical fiber, comprising a tetrafluorosilane gas obtained by theproduction process described above.

[0017] The present invention further provides a gas for the productionof a semiconductor, comprising a tetrafluorosilane gas obtained by theproduction process described above.

[0018] The present invention further provides a gas for the productionof a solar cell, comprising a tetrafluorosilane gas obtained by theproduction process described above.

BRIEF DESCRIPTION OF DRAWING

[0019]FIG. 1 is a schematic view of an apparatus usable for theproduction process of tetrafluorosilane of the present invention.

[0020] In the drawing, 1 denotes a thermal decomposition reactor, 2 adecomposition reaction tube, 3 an electric furnace, 4 a heater, 5hexafluorosilicate, 6 a thermometer, 7 an Ni porous plate for fixing, 8an F₂ reactor (filled or not filled with highvalent metal fluoxide), 9 areactor (silicon), 11 a gas separation membrane module, 12, 36 vacuumpumps, 13, 14, 21 pressure gauges, 15 a separation membrane by-passline, 16 a recovery container, 19 an adsorption tower, 22 to 25 flowrate controlling valves, 26 a pressure regulator, 27 to 32 samplingvalves, and 33 to 35 recovery container valves.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] The present invention is described in detail below.

[0022] The hexafluorosilicate is preferably at least one compoundselected from the group consisting of alkali metal hexafluorosilicateand alkaline earth metal hexafluorosilicate. Examples of these compoundsinclude Li₂SiF₆, Na₂SiF₆, K₂SiF₆, Cs₂SiF₆, MgSiF₆, CaSiF₆, SrSiF₆ andBaSiF₆. These compounds all are available as an industrial product at alow cost and, in the production process of the present invention, thesecompounds may be used individually or in combination of two or morethereof. Among these, Na₂SiF₆ (sodium hexafluorosilicate) obtained as aby-product in the process of producing phosphoric acid is preferred, inview of the cost, because this compound is mass-produced.

[0023] For example, in the case of using Na₂SiF₆ (sodiumhexafluorosilicate) obtained in the process of producing phosphoricacid, the Na₂SiF₆ crystals are a crystalline powder of from tens of μmto hundreds of μm and sometimes contains about 10% by mass of water.Accordingly, in the production process of the present invention forproducing tetrafluorosilane by using a hexafluorosilicate as a startingmaterial, the hexafluorosilicate is preferably pulverized and driedbefore conducting the step (1). By pulverizing the hexafluorosilicate,the surface area of the hexafluorosilicate crystals increases and thisis considered to facilitate the drying of the crystal.

[0024] The pulverization of the hexafluorosilicate crystal may beperformed by using a pulverizer such as a ball mill and the crystal ispulverized to a particle size of 100 μm or less, preferably 10 μm orless, more preferably 1 μm or less. Then, while passing nitrogen, air orthe like, the crystal is dried. Here, the decomposition initiationtemperature differs depending on the kind of hexafluorosilicate andtherefore, the drying temperature is preferably selected from thetemperatures lower than the decomposition initiation temperature. Forexample, in the case of drying sodium hexafluorosilicate, the crystal ispreferably dried at a temperature of 200° C. to less than 400° C., morepreferably from 300° C. to less than 400° C.

[0025] The purpose of performing the drying step after the pulverizationof the crystals is to reduce the amounts of impurities such as HF and(SiF₃)₂O generated as by-products due to reaction of SiF₄, produced inthe step (1), with water. For example, in the case of using a sodiumsalt as the hexafluorosilicate, SiF₄ is produced according to thefollowing formula (1) and it is considered that if water is present atthis time, HF and (SiF₃)₂O are produced according to the followingformula (2):

Na₂SiF₆→SiF₄+2NaF  (1)

2SiF₄+H₂O→(SiF₃)₂O+2HF  (2)

[0026] In the production process of tetrafluorosilane of the presentinvention, even if HF and (SiF₃)₂O are contained in SiF₄ at the stage ofconducting the step (1), these can be treated in a subsequent step andthere arises no problem, however, by pulverizing and drying thehexafluorosilicate crystal before conducting the step (1), theproduction amount of (SiF₃)₂O can be reduced to ⅓ to ⅕. Here, the reasonwhy the production of (SiF₃)₂O cannot be completely prevented is becausetrace amounts of water and oxygen-containing silicic acid compounds(e.g., SiO₂) remain in the crystal. The reduction in the productionamount of (SiF₃)₂O is advantageous also in view of profitability,because the amount of fluorine gas (hereinafter sometimes referred to as“F₂”) added, for example, in the step (2) can be reduced. Also, thehexafluorosilicate crystal is preferably dried at a temperature of 50 to200° C. in advance of pulverizing the crystal so as to facilitate thepulverization.

[0027] In the production process of tetrafluorosilane of the presentinvention, the respective operations of drying and pulverizinghexafluorosilicate and drying the pulverized crystal are preferablyconducted before the step (1) but when a hexafluorosilicate having a lowwater content can be used, these operations need not be conducted.However, complete removal of water included in the hexafluorosilicatecrystal is very difficult and the drying temperature has an upper limitbecause if the drying temperature is elevated, there arises a problemthat the production of SiF₄ starts. Therefore, it is very difficult tocompletely prevent the production of HF and (SiF₃)₂O, which isascribable to water contained in the crystal. Furthermore, theoxygen-containing silicic acid compound (e.g., SiO₂) cannot be removedby the heat treatment and causes the production of (SiF₃)₂O.

[0028] The step (1) is a step of heating a hexafluorosilicate to produceSiF₄. The step (1) may be conducted in an inert gas stream, for example,nitrogen gas, or in a vacuum. The preferred range of heating temperaturecan be selected according to the hexafluorosilicate employed. Forexample, in the case of using a barium salt, the heating is preferablyconducted at a temperature of 400 to 700° C. and in the case of using asodium salt, the heating is preferably conducted at a temperature of 500to 800° C.

[0029] In the production process of tetrafluorosilane of the presentinvention, the step (2-1), the step (2-2), or the steps (2-1) and (2-3)are conducted subsequent to the step (1). The step (2-1) is a step ofreacting a mixed gas containing SiF₄ and (SiF₃)₂O, produced in the step(1), with a fluorine gas. The reaction temperature is preferably from100 to 350° C., more preferably from 200 to 350° C. The F₂ gas issuitably reacted in an equimolar amount to 2 molar times to theproduction amount of (SiF₃)₂O contained in the SiF₄ produced. Thereaction in the step (2) is considered to proceed according to thefollowing formula (3), whereby the (SiF₃)₂O can be converted into SiF₄and O₂:

(SiF₃)₂O+F₂→2SiF₄+½O₂  (3)

[0030] If the amount of F₂ gas exceeds 2 molar times the productionamount of (SiF₃)₂O, the effect is saturated and this is not preferred inview of profitability. In considering the corrosion resistance of theconstruction material of the reactor against F₂ gas, the reactiontemperature is preferably 350° C. or less.

[0031] The step (2-2) is a step of reacting the SiF₄ gas containing(SiF₃)₂O, which is produced in the step (1), with a highvalent metalfluoxide. The step (2-3) is a step of reacting the SiF₄ gas produced inthe step (2-1) with a highvalent metal fluoxide. The reaction in thestep (2-2) or the step (2-3) is the decomposition ofhexafluorodisiloxane by the highvalent metal fluoxide and thehexafluorodisiloxane decomposes upon reaction with the highvalent metalfluoxide to produce SiF₄ and O₂.

[0032] Examples of the highvalent metal fluoxide which can be used inthe step (2-2) or the step (2-3) include compounds such as CoF₃, MnF₃,MnF₄, AgF₂, CeF₄, PbF₄ and K₃NiF₇. These compounds have a property ofactivating fluorine when heated and, due to the activated fluorine, thedecomposition reaction of hexafluorodisiloxane is considered to proceedaccording to the reactions of the following formulae (4) to (10):

2CoF₃+(SiF₃)₂O→2CoF₂+2SiF₄+½O₂  (4)

2MnF₃+(SiF₃)₂O→2MnF₂+2SiF₄+½O₂  (5)

MnF₄+(SiF₃)₂O→MnF₂+2SiF₄+½O₂  (6)

2AgF₂+(SiF₃)₂O→2AgF+2SiF₄+½O₂  (7)

2CeF₄+(SiF₃)₂O→2CeF₃+2SiF₄+½O₂  (8)

PbF₄+(SiF₃)₂O→PbF₂+2SiF₄+½O₂  (9)

2K₃NiF₇+(SiF₃)₂O→2K₃NiF₆+2SiF₄+½O₂  (10)

[0033] Of course, these highvalent metal fluoxides may be usedindividually or as a mixture.

[0034] The preparation method of a highvalent metal fluoxide isdescribed below by referring to the case of a highvalent metal fluoridewhere CoF₃ is supported on a support. For example, Co(NO₃)₂.6H₂O isdissolved in water, the resulting aqueous solution is absorbed to dryAl₂O₃ (NST-3, produced by Nikki Kagaku K.K.), and this is dried on awarm bath until the water content becomes nil. After the drying, thealumina is filled into a nickel tube and baked in a N₂ stream, therebyremoving water and nitric acid residue, to obtain an oxide.Subsequently, by passing a 10% F₂ (N₂ dilution) gas, fluorination of Coand alumina used as a support is performed.

[0035] In the case of using alumina, titania, zirconia or the like as aformation aid, as such, oxygen on the support surface and SiF₄ react toproduce hexafluorodisiloxane and therefore, it is necessary to performthorough fluorination before the passing of SiF₄. The fluorination ofsupport can be easily performed by passing fluorine or HF gas in theheated state. By finally treating with a fluorine gas before use, anobjective highvalent metal fluoxide can be obtained.

[0036] The step (2-2) or the step (2-3) is preferably conducted at atemperature of 50 to 350° C., more preferably from 150 to 350° C. Whenthe highvalent metal fluoxide in the heated state is passed to a mixedgas of hexafluorodisiloxane and SiF₄, the hexafluorodisiloxanedecomposes to produce SiF₄ and O₂. At this time, if the linear velocityis too high, the break-through zone becomes long and the life isshortened, therefore, the passing is preferably conducted at 10 m/min orless in terms of a linear velocity at ordinary temperature underatmospheric pressure.

[0037] If the reaction of the step (2-2) or the step (2-3) is continued,the highvalent metal fluoxide becomes a normal valent metal fluoride andloses the fluorination capability, and hexafluorodisiloxane is detectedat the reactor outlet. In this case, it may be possible to stop thereaction and refluorinate the low-order fluoride with a fluorine gasinto a highvalent metal fluoxide but, in order to continuously conductthe reaction, it may also be possible to use two or more reaction towersand while repeating the cycle of reaction and regeneration, continuouslyconduct the decomposition reaction. The timing for the switch-over canbe confirmed by carrying out hexafluorodisiloxane analysis of thereactor outlet gas by FT-IR.

[0038] The step (2-2) or the step (2-3) is preferably conducted in thepresence of a fluorine gas and by passing a fluorine gas while heating,the reaction can be continuously conducted while regenerating thehighvalent metal fluoxide according to the reactions of the followingformulae (11) to (17).

CoF₂+½F₂→CoF₃  (11)

MnF₂+½F₂→MnF₃  (12)

MnF₂+F₂→MnF₄  (13)

AgF+½F₂→AgF₂  (14)

CeF₃+½F₂→CeF₄  (15)

PbF₂+F₂→PbF₄  (16)

K₃NiF₆+½F₂→K₃NiF₇  (17)

[0039] The tetrafluorosilane containing hexafluorodisiloxane as animpurity is mixed with a fluorine gas in an equimolar amount tohexafluorodisiloxane and passed to a highvalent metal fluoxide, wherebythe decomposition reaction of hexafluoro-disiloxane by the highvalentmetal fluoxide and the regeneration of normal valent metal fluoride bythe fluorine gas can be simultaneously attained. At this time, the spacevelocity is 10,000 hr⁻¹ or less, preferably 5,000 hr⁻¹ or less, morepreferably 1,000 hr⁻¹, at ordinary temperature under atmosphericpressure. The amount of inlet fluorine gas can be controlled by feedingan equimolar amount of fluorine gas while analyzing the amount ofhexafluorodisiloxane at the inlet of reactor by FT-IR.

[0040] The tetrafluorosilane gas obtained through the step (2-1), thestep (2-2), or the steps (2-1) and (2-3) sometimes contains fluorine gasadded in excess. Accordingly, in the production process oftetrafluorosilane of the present invention, a step (3) of contactingsilicon with the tetrafluorosilane gas containing fluorine gas ispreferably conducted after the step (2-1), the step (2-2), or the steps(2-1) and (2-3).

[0041] The step (3) for converting excess fluorine gas into SiF₄ ispreferably performed at a temperature of 50° C. or more, more preferably100° C. or more, still more preferably 150° C. or more. The silicon usedin the step (3) is preferably silicon where the hydroxyl group on thesilicon surface is heat-treated using an inert gas such as nitrogen gasat a temperature of 400° C. or more, preferably from 400 to 600° C.

[0042] In the step (3), it is not preferred to use SiO₂ in place ofsilicon because, due to a reaction with HF contained in SiF₄, H₂O isproduced and, furthermore, (SiF₃)₂O is produced according to thefollowing reactions of formulae (18) and (19):

SiO₂+4HF→SiF₄+2H₂O  (18)

2SiF₄+H₂O→(SiF₃)₂O+2HF  (19)

[0043] The F₂ gas starts reacting from the surface of silicon andtherefore, although the shape of silicon, such as particle size andsurface area, is not particularly limited, a chip having a particle sizeon the order of several mm is preferred in considering the permeabilityof gas, the contact property or the filling operation. The purity ofsilicon chip is preferably 99.9% by mass or more, more preferably99.999% by mass or more, and most preferably or a semiconductor siliconwafer grade.

[0044] The production process of tetrafluorosilane of the presentinvention preferably contains a step (4) of contacting the gas obtainedthrough the step (2-1), the step (2-2), the steps (2-1) and (2-3), thesteps (2-1) and (3), the steps (2-2) and (3), or the steps (2-1), (2-3)and (3) with a gas separation membrane and/or a molecular sievingcarbon.

[0045] The gas separation membrane is preferably an SiO₂—ZrO₂ ceramicmembrane and/or a poly(4-methylpentene-1) heterogeneity membrane. Themolecular sieving carbon preferably has a pore size of 5 Å or less.

[0046] The SiF₄ produced by the production process of the presentinvention may contain impurities produced in respective steps describedabove. Examples of the impurities include (SiF₃)₂O, H₂, O₂, N₂ and HF.Also, impurities such as CO and CO₂, considered to be originated in aslight amount of a carbon compound present in the raw materialhexafluorosilicate, may be contained. In order to obtain high-puritySiF₄, these impurities are preferably separated by purification.

[0047] In the production process of tetrafluorosilane of the presentinvention, SiF₄ containing impurities such as O₂, N₂, CO, CO₂ and HF is,for example, is contacted with a gas separation membrane and/or amolecular sieving carbon to separate O₂, N₂, CO, CO₂, HF and the likefrom SiF₄, whereby high-purity SiF₄ can be obtained.

[0048] Examples of the gas separation membrane which can be used includeseparation membrane Module SiO₂—ZrO₂ Membrane (dimension of module:φ50×300L) produced by Kyocera Corporation, and a poly(4-methylpentene-1)heterogeneity membrane (dimension of module: φ60×500L) produced byDai-Nippon Ink & Chemicals, Inc. These separation membranes may be usedindividually or in combination.

[0049] The gas separation membrane for use in the production process oftetrafluorosilane of the present invention is not limited to theabove-described separation membranes insofar as the separation membranehas a large permeation (separation) coefficient of SiF₄ and impuritiessuch as O₂, N₂, CO, CO₂ and HF.

[0050] The molecular sieving carbon is not limited to theabove-described molecular sieving carbon insofar as the molecularsieving carbon has a pore size large enough to adsorb the impuritiessuch as O₂, N₂, CO, CO₂ and HF and a pore size small enough not to allowthe adsorption of SiF₄. The pore size is preferably 5 Å or less becauseO₂, N₂, CO, CO₂ and HF are adsorbed and SiF₄ is not adsorbed.

[0051] The method of purifying an SiF₄ gas using a gas separationmembrane module is described below.

[0052] In the method of purifying an SiF₄ gas using a gas separationmembrane module, the gas separation membrane module is previously purgedwith N₂ gas or the like to remove H₂O which reacts with SiF₄. Thepurging is regarded as completed when the supply N₂ gas, thenon-permeated side and the permeated side reach the same gas dew point.The N₂ gas for drying is not particularly limited insofar as the dewpoint is −70° C. or less.

[0053] The supply side of the dried gas separation membrane module iscontacted with SiF₄ containing impurities such as O₂, N₂, CO, CO₂ and HFto selectively permeate the impurities such as O₂, N₂CO, CO₂ and HF,whereby SiF₄ is concentrated in the non-permeated side and a high-puritySiF₄ can be obtained. The SiF₄ concentrated in the non-permeated sidemay be further contacted with a molecular sieving carbon.

[0054] In the method of separating SiF₄ from O₂, N₂, CO, CO₂, HF and thelike using a gas separation membrane module, as the pressure differencebetween the permeated side and the non-permeated side is larger, SiF₄having a higher purity can be obtained in the non-permeated side ofmembrane, therefore, the non-permeated side (supply side) of membrane ispressurized to atmospheric pressure or more. In addition, if desired,the permeated side of membrane may be depressurized to atmosphericpressure or less.

[0055] The method of purifying an SiF₄ gas using a molecular sievingcarbon is described below.

[0056] Examples of the molecular sieving carbon (hereinafter sometimesreferred to as “MSC”) which can be used include MORSIEBON 4A (tradename) produced by Takeda Chemical Industries, Ltd. In the purificationmethod using an adsorbent, MSC is filled in a container and, in advance,is preferably baked with an inert gas such as N₂ at a temperature of 100to 350° C. so as to remove water, CO₂ and the like adsorbed to theadsorbent. The baking may be performed by N₂ purging under heat invacuum. The baking can be regarded as completed when the supply gas andthe discharge gas reach the same dew point. The N₂ for drying is notparticularly limited insofar as the dew point is −70° C. or less.

[0057] By contacting the above-described MSC with the SiF₄ gascontaining impurities such as O₂, N₂, CO, CO₂ and HF, obtained by theproduction process of the present invention and thereby allowing onlythe impurity gases such as O₂, N₂, CO, CO₂ and HF to adsorb to the MSC,high-purity SiF₄ can be obtained.

[0058] The adsorption of impurity gases such as O₂, N₂, CO, CO₂ and HFcontained in SiF₄ to MSC is preferably conducted, according to a generaladsorption separation purifying method, by setting the adsorptiontemperature to a lower temperature and the adsorption pressure to ahigher pressure. In the case of conducting the adsorption at ordinarytemperature, the pressure is atmospheric pressure or more, preferably0.5 MPa or more, more preferably 1 MPa or more. In the case of adsorbingthe impurity gases while cooling, the pressure is preferably lower thanthe SiF₄ liquefaction pressure.

[0059] The linear velocity (LV, m/min) in terms of a linear velocityunder atmospheric pressure is suitably 5 or less, preferably 2 or less,more preferably 1 or less. The space velocity (SV, H⁻¹) is 1,000 orless, preferably 500 or less, more preferably 200 or less.

[0060] When two adsorption towers are used, SiF₄ can be continuouslypurified by alternately conducting the adsorption and the regeneration.The regeneration can be carried out, for example, by exhausting a partof SiF₄ in the purification adsorption tower under heat in vacuum to theregeneration desorption tower while purging from the direction reversedto the adsorption purification.

[0061] The production process of tetrafluorosilane of the presentinvention is also characterized by using the analysis method describedlater for the process control.

[0062] The tetrafluorosilane obtained by the production process oftetrafluorosilane of the present invention may be a high-puritytetrafluorosilane having a content of hexafluorodisiloxane contained asan impurity of 1 vol ppm or less. A high-purity tetrafluorosilane havinga hexafluorodisiloxane content of 0.1 vol ppm or less can also beobtained.

[0063] The method for analyzing impurities in a high-puritytetrafluorosilane of the present invention is described below. As forthe numerical values described later, these are of course notparticularly limited.

[0064] The method for analyzing impurities in a high-puritytetrafluorosilane of the present invention is characterized by bringingtetrafluorosilane containing an H₂ gas, an O₂ gas, an N₂ gas, a CO gas,a CH₄ gas and/or a CO₂ gas as impurities into contact with an adsorbentto separate the impurities from tetrafluorosilane, and introducing theimpurities together with a carrier gas into a gas chromatograph toanalyze the impurities.

[0065] The components which can be analyzed by the analysis method ofthe present invention are trace H₂, O₂, N₂, CO, CH₄ and/or CO₂ gases.Also, the components F₂, HF and (SiF₃)₂O can be analyzed.

[0066] The adsorbent is preferably an activated carbon, a petroleumpitch spherical activated carbon and/or a molecular sieving carbonhaving a pore size of 6 Å or more.

[0067] According to the analysis method of the present invention, apre-column (an SUS column of φ3 mm (inner diameter)×1 m (length)) packedwith SHINCARBON—S (activated carbon adsorbent, produced by ShimadzuCorporation) of 60 to 100 mesh is fixed in a constant temperature bathoven and kept at 100° C. Into this pre-column, 1 ml of an SiF₄ gascontaining impurities such as H₂, O₂, N₂, CO, CH₄, CO₂, HF and (SiF₃)₂Ois introduced as a sample through a cock with a gas sampler. For thecarrier gas, a high-purity helium (He) gas can be used.

[0068] In the sample accompanied by a high-purity He carrier gas, theH₂, O₂, N₂, CO, CH₄ and CO₂ gases are separated in the pre-column andSiF₄, HF and (SiF₃)₂O are adsorbed to the pre-column. Of the separatedimpurity gases, H₂, O₂, N₂, CO and CH₄ can be separated using aseparation column, for example, Molecular Sieve 5A (trade name). In thecase of containing CO₂, this can be separated using a separation column,for example, POLAPACK Q (trade name).

[0069] Respective components separated are subsequently introduced intoa PDD (pulsed discharge detector) and measured on each gasconcentration. The detection limit of impurity gases such as H₂, O₂, N₂,CO, CH₄ and CO₂ is 0.01 vol ppm. According to the analysis method of thepresent invention, a quantitative analysis can be carried out to aconcentration of 0.05 to 0.1 vol ppm and, thus, high-purity SiF₄ can beanalyzed.

[0070] In the analysis method of the present invention, theabove-described activated carbon is used for the pre-column because itscapability of separating the sample into a group of components H₂, O₂,N₂, CO, CH₄ and CO₂ and a group of main component SiF₄ and impurities HFand (SiF₃)₂O is excellent as compared with other adsorbents such assilica gel, zeolite and porous polymer beads. The petroleum pitchactivated carbon is preferred because the ash content (e.g., K₂CO₃) isvery small as compared with activated carbon and good separation of maincomponent SiF₄ can be attained. The molecular sieving carbon can attainexcellent separation of main component SiF₄ as compared with those ACand BAC and is more preferred. The reason therefor is considered becausethe pore size and the distribution thereof are controlled.

[0071] On the other hand, the HF, (SiF₃)₂O and SiF₄ remaining adsorbedto the pre-column can be exhausted and regenerated by employing abackflash system of changing over the flow path of high-purity Hecarrier gas using a cock and purging the pre-column from the directionreverse to the direction at the introduction of sample. At this time,the temperature in the constant temperature bath oven at 100° C. wherethe pre-column is fixed can be elevated to 200° C. simultaneously withthe changeover of cock so as to accelerate the exhaust of HF, (SiF₃)₂Oand SiF₄. The aging temperature of the pre-column and separation columnmay be a maximum temperature commonly used plus about 50° C.

[0072] The method for analyzing impurities in a high-puritytetrafluorosilane of the present invention is characterized byintroducing tetrafluorosilane containing hexafluorodisiloxane as animpurity into a cell with the material of window being composed of ametal halide, and analyzing the hexafluorodisiloxane and/or hydrogenfluoride by infrared spectrometry.

[0073] In the analysis method of the present invention, infraredspectrometry is used, whereby the concentration of hexafluorodisiloxanecontained in tetrafluorosilane can be measured and in addition, theconcentrations of hydrogen fluoride (HF) can also be measured.

[0074] In the analysis of (SiF₃)₂O, a standard gas of (SiF₃)₂O isdifficult to prepare and, therefore, the (SiF₃)₂O content may bedetermined, for example, from the absorbancy ratio {(SiF₃)₂O/SiF₄} ofthe infrared absorption peculiar to (SiF₃)₂O at 838 cm⁻¹ to the infraredabsorption peculiar to SiF₄ at 2,054 cm⁻¹. In this case, a value shownin the literature (for example, Anal. Chem., 57, 104-109 (1985)) can beused as the standard for the absorbancy of (SiF₃)₂O.

[0075] SiF₄ containing (SiF₃)₂O is introduced into a gas cell having along optical path, for example, in a length of 4 m or more and infraredspectrometry is used, whereby the concentration of (SiF₃)₂O in SiF₄ canbe analyzed to 0.1 ppm or less. The infrared spectrometer is preferablya Fourier transform-type infrared spectrometer.

[0076] In the analysis method of the present invention, (SiF₃)₂O in alow concentration can be analyzed by measuring the infrared absorptionspectrum at 838 cm⁻¹ peculiar to (SiF₃)₂O.

[0077] Also, the (SiF₃)₂O concentration in SiF₄ can be indirectlymeasured by adding a constant excess amount of F₂ to a constant amountof SiF₄ containing (SiF₃)₂O, reacting (SiF₃)₂O and F₂ under heating at300° C., and determining the consumption (excess) of F₂.

[0078] In the analysis method of the present invention, the measurementis preferably performed by FT-IR where at least the portion coming intocontact with SiF₄ in the sampling line is composed of a stainless steelor an electropolished stainless steel and the optical transmissionwindow of the gas cell is formed of a construction material of KCl,AgCl, KBr or CaF₂. HF can be analyzed to 0.1 ppm or less from theabsorbancy at 4,040 cm⁻¹ peculiar to HF in the infrared absorptionspectrum in the same manner as (SiF₃)₂O.

[0079] Uses of the high-purity tetrafluorosilane obtained by the processof the present invention are described below.

[0080] When the integration degree of transistors is elevatedaccompanying the refinement of semiconductor devices, the integrationdensity or the switching speed of individual transistors can beadvantageously increased. However, the propagation delay due to wiringcancels the improvement in the speed of transistor. In the generation ofa line width of 0.25 μm or more, the delay due to wiring is a seriousproblem. In order to solve this problem, aluminum is replaced by copperwiring as a low-resistance wiring and a low dielectric interlayerinsulating film is employed so as to reduce the capacitance betweenwirings. The representative low dielectric material employed in thegeneration for a line width of 0.25 to 0.18 or 0.13 μm includes SiOF(fluorine-doped oxide film, ε: around 3.5) formed by HDP (high density)plasma CVD. A process using SiOF for the interlayer insulating film andan aluminum alloy for the wiring is proceeding and the high-purity SiF₄of the present invention can be used as a doping material therefor.

[0081] The glass for an optical fiber comprises a core part and a cladpart. The core part is rendered to have a higher refractive index thanthe peripheral clad part so as to more easily transmit the light in thecenter part. The refractive index can be made higher by adding Ge, Al,Ti or the like as a dopant. However, this has an adverse effect in thatthe light scattering increases due to the dopant and the lighttransmission efficiency decreases. When fluorine is added to the cladpart, the refractive index can be made lower than pure quartz andtherefore, pure quartz or quartz reduced in the dopant can be used forthe core part to increase the light transmission efficiency. Thefluorine is added by heat-treating a glass fine particle material (SiO₂)in He in an atmosphere of SiF₄ and the high-purity SiF₄ of the presentinvention can be used as the gas for an optical fiber.

[0082] The present invention is further illustrated below by referringto Examples, however, the present invention is not limited to theseExamples.

EXAMPLE 1

[0083] Sodium hexafluorosilicate (Na₂SiF₆) 5 having an average particlesize of about 70 μm and a purity of 89% by mass or more (water content:10% by mass or less), obtained as a by-product in the production processof phosphoric acid was dried by a hot air dryer at 120° C., 1,500 gthereof was filled into the center of a decomposition reaction tube 2(inner diameter: 90 mm, length: 1,500 mm, construction material: nickel)of a thermal decomposition reactor 1 shown in FIG. 1, and both ends wereclosed by an Ni porous plate 7. Then, while controlling the temperatureof Na₂SiF₆ to less than 400° C. by an electric furnace 3 (length: 1,000mm), N₂ gas (dew point: 70° C. or less) was passed at 1,000 ml/min, byopening the valve 22, and when the reduction of the HF concentration inthe exhaust gas to 1 ppm or less was confirmed, the drying of Na₂SiF₆was completed. Thereafter, the flow rate of N₂ gas was controlled to 200ml/min and the temperature of the electric furnace 3 was elevated to700° C. and kept at 700° C. As a result, SiF₄ having a concentration ofabout 30 vol % was generated. The gas generated was sampled from thevalve 27 and the concentrations of impurity gases were analyzed. Theresults are shown in Table 1. From the results, it is seen that 8,560ppm of (SiF₃)₂O was contained.

EXAMPLE 2

[0084] SiF₄ gas was generated in the same manner as in Example 1 exceptthat the dry sodium hexafluorosilicate crystal used in Example 1 waspulverized by a pulverizer and the obtained powder having a particlesize of about 1 μm was filled in the decomposition reaction tube 2. Thegas generated was sampled from the valve 27 and the concentrations ofimpurity gases were analyzed. The results are shown in Table 1. From theresults, it is seen that the (SiF₃)₂O concentration was reduced whenpulverization was performed.

EXAMPLE 3

[0085] The SiF₄ gas generated in Example 2 was introduced in an F₂reactor 8 (construction material of reaction tube: nickel, innerdiameter: 8 mm, length: 1,000 mm) shown in FIG. 1, 3 ml of 100% fluorinegas was mixed through the valve 23, and (SiF₃)₂O contained in SiF₄ wasreacted with F₂ at 300° C. The gas generated was sampled from the valve28 and analyzed. The values obtained are shown in Table 1. From theresults, it is seen that the (SiF₃)₂O concentration was reduced to lessthan 0.1 vol ppm.

EXAMPLE 4

[0086] Into a reaction tube (construction material: nickel) of a reactor9 shown in FIG. 1, 60 ml of silicon chips having a size of 8 to 10 meshwere filled and treated at 500° C. for 3 hours while flowing N₂ gas (dewpoint: −70° C. or less) at 300 ml/min. Then, the reaction tube filledwith silicon chips was kept at a temperature of 150° C. and the gasobtained in Example 3 was introduced thereinto to react excess F₂ gasand silicon. The gas generated was sampled from the valve 29 andanalyzed. The values obtained are shown in Table 1. From the results, itis seen that the fluorine gas concentration was decreased to less than0.1 vol ppm.

EXAMPLE 5

[0087] Into a gas separation membrane module 11 (SiO₂—ZrO₂ membrane,produced by Kyocera Corporation) shown in FIG. 1, N₂ gas (dew point:−70° C. or less) was passed at 2 to 3 L/min and dried until the inletand the outlet reached the same dew point. At the dried gas separationmembrane module 11, the gas obtained in Example 4 was introduced intothe supply side at an atmospheric pressure and while depressurizing thepermeated side by a dry vacuum pump 12, impurity gases such as N₂ wereseparated to the permeated side. The gas in the permeated side wassampled from the valve 30 and the gas in the non-permeated side wassampled from the valve 31. These gases were analyzed. The analysisresults obtained are shown in Table 1. From the results, it is seen thatmost of impurity gases could be removed to the permeated side.

[0088] The SiF₄ in the non-permeated side was freeze-recovered in arecovery container 16 cooled to −120° C. with liquid N₂, through thevalves 24 and 33 while controlling the pressure.

EXAMPLE 6

[0089] Into an adsorption tower 19 (inner diameter: 16 mm, length: 1,000mm) shown in FIG. 1, 100 ml of MSC (MORSIEBON 4A, produced by TakedaChemical Industries, Ltd.) was filled and while passing N₂ gas (dewpoint: −70° C. or less) at 500° C. and 300 ml/min, dried until the inletand the outlet reached the same dew point. After cooling and thenpurging with He, SiF₄ recovered in the recovery container 16 in Example5 was gasified at an ordinary temperature and introduced into theadsorption tower 19. At this time, the pressure was regulated to 0.9 MPaand the flow rate of SiF₄ gas was controlled to 350 ml/min, using a flowrate controlling valve 25, a pressure regulator 26 and a pressure gauge21. The outlet gas of the adsorption tower 19 was analyzed on theimpurity gases in the same manner and the analysis values are shown inTable 1. From the results, it is seen that the concentrations of allimpurities measured were less than 0.1 vol ppm.

EXAMPLE 7

[0090] The gas obtained in Example 4 was freeze-recovered in a recoverycontainer 16 cooled to −120° C. with liquid N₂, through valves 24 and 33while regulating the pressure using a gas separation membrane by-passline 15 shown in FIG. 1. Then, while gasifying at an ordinarytemperature, the SiF₄ recovered in the recovery container 16 wasintroduced into the adsorption tower 19 treated in the same manner as inExample 6, under the same conditions as in Example 6. The gas introducedinto the adsorption tower 19 and the outlet gas from the adsorptiontower 19 were analyzed and the results are shown in Table 1. From theresults, it is seen that the concentrations of measured all impuritiesin the outlet gas were less than 0.1 vol ppm.

EXAMPLE 8

[0091] Preparation of Highvalent Metal Fluoxide Supported on Support:10% CoF₃/Al₂O₃

[0092] In 200 ml of water, 26.4 g (0.0091 mol) of Co(NO₃)₂.6H₂O [extrapure reagent] was dissolved. The resulting aqueous solution was absorbedinto 100.2 g of dry Al₂O₃ (NST-3, produced by Nikki Kagaku K.K.), andthis was dried on a warm bath until the water content became nil. Afterthe drying, the alumina was filled into a reaction tube (constructionmaterial: nickel) of a reactor 8 shown in FIG. 1 and baked at 400° C.for 12 hours in a N₂ stream (400 ml/min), thereby removing water andnitric acid residue, to obtain an oxide of Co. Subsequently, a 10% F₂(N₂ dilution) gas was passed (1,000 ml/min) at 250° C. to perform thefluorination of alumina and Co. The fluorination was performed until thereactor inlet fluorine concentration and the outlet fluorineconcentration became the same. The concentration was measured by passinga gas to be analyzed through an aqueous 5% KI solution and titrating theliberated 12 with an aqueous 0.1N-Na₂S₂O₃ solution.

[0093] Using 100 ml of highvalent metal fluoxide prepared above,decomposition of hexafluorodisiloxane was performed. The SiF₄ gasgenerated in Example 2 was introduced into a reactor 8 (constructionmaterial of reaction tube: nickel, inner diameter: 8 mm, length: 1,000mm) shown in FIG. 1, and (SiF₃)₂O contained in SiF₄ was reacted withhighvalent metal fluoxide (CoF₃) at 200° C. The gas generated wassampled from the valve 28 and analyzed. The values obtained are shown inTable 1. From the results, it is seen that (SiF₃)₂O was reduced to lessthan 0.1 vol ppm. The reactor outlet analysis was continued and, then,(SiF₃)₂O was detected from the outlet.

EXAMPLE 10

[0094] 100 ml of highvalent metal fluoxide prepared in Example 8 wasused. The SiF₄ gas generated in Example 1 was introduced into a reactor8 (construction material of reaction tube: nickel, inner diameter: 8 mm,length: 1,000 mm) shown in FIG. 1, 2.3 ml of 100% fluorine gas was mixedthrough the valve 23 and while reacting (SiF₃)₂O contained in SiF₄ withhighvalent metal fluoxide at 250° C., the highvalent metal fluoxide wasregenerated by F₂. The gas generated was sampled from the valve 28 andanalyzed. The values obtained are shown in Table 1. From the results, itis seen that (SiF₃)₂O was reduced to about 500 vol ppm. The reactoroutlet analysis was continued, however, the fluorine gas was notdetected from the outlet and the (SiF₃)₂O concentration was not changed.TABLE 1 Analysis Results of Respective Components (vol ppm) (SiF₃)₂O N₂O₂ CO CO₂ HF F₂ Example 1 8560 — — — — — — Example 2 1850 — — — — — —Example 3 <0.1 — 30.0 5.0 83.0 5.5 1680 Example 4 <0.1 — 29.8 4.7 83.65.6 <0.1 Example 5 permeated <0.1 — 75.2 12.3 212 14.2 — side non- <0.188.7 0.5 <0.1 0.5 <0.1 — permeated side Example 6 <0.1 <0.1 <0.1 <0.1<0.1 <0.1 — Example 7 inlet <0.1 48.5 1.5 1.7 187 9.3 — outlet <0.1 <0.1<0.1 <0.1 <0.1 <0.1 — Example 8 0.5 hr <0.1 — — — — — — 2.0 hr 102 — — —— — — Example 9 0.5 hr 460 — — — — — <0.1 4.0 hr 480 — — — — — <0.1

INDUSTRIAL APPLICABILITY

[0095] As described in the foregoing, according to the presentinvention, SiF₄ not containing (SiF₃)₂O can be produced. Also, impuritycomponents can be analyzed to 0.1 ppm or less and high-purity SiF₄,required in the production of electronic parts, can be provided.

1. A process for producing tetrafluorosilane, comprising a step (1) ofheating a hexafluorosilicate, a step (2-1) of reacting atetrafluorosilane gas containing hexafluorodisiloxane produced in thestep (1) with a fluorine gas, a step (2-2) of reacting atetrafluorosilane gas containing hexafluorodisiloxane produced in thestep (1) with a highvalent metal fluoxide, or a step (2-1) of reacting atetrafluorosilane gas containing hexafluorodisiloxane produced in thestep (1) with a fluorine gas and a step (2-3) of reacting atetrafluorosilane gas produced in the step (2-1) with a highvalent metalfluoxide.
 2. A process according to claim 1, wherein the step (1) isconducted at a temperature of 400° C. or more.
 3. A process according toclaim 1, wherein the step (2-1) is conducted at a temperature of 100 to350° C.
 4. A process according to claim 1, wherein the step (2-2) or thestep (2-3) is conducted at a temperature of 50 to 350° C.
 5. A processaccording to any one of claims 1 to 4, wherein the hexafluorosilicate isat least one compound selected from the group consisting of alkali metalhexafluorosilicate and alkaline earth metal hexafluorosilicate.
 6. Aprocess according to any one of claims 1 to 5, wherein thehexafluorosilicate is pulverized and dried before conducting the step(1).
 7. A process according to any one of claims 1 to 6, wherein thehighvalent metal fluoxide is at least one compound selected from thegroup consisting of CoF₃, MnF₃. MnF₄, AgF₂, CeF₄, PbF₄ and K₃NiF₇.
 8. Aprocess according to any one of claims 1 to 7, wherein the highvalentmetal fluoxide is supported on a support.
 9. A process according toclaim 8, wherein the support is obtained by fluorinating at least onemember selected from the group consisting of alumina, titania andzirconia.
 10. A process according to any one of claims 1 to 9, whereinthe step (2-2) or the step (2-3) is conducted in the presence of afluorine gas.
 11. A process according to any one of claims 1 to 10,which comprises a step (3) of contacting silicon with thetetrafluorosilane gas obtained through the step (2-1), the step (2-2),or the steps (2-1) and (2-3).
 12. A process according to claim 11,wherein the step (3) is conducted at a temperature of 50° C. or more.13. A process according to claim 11 or 12, wherein the silicon isheat-treated at a temperature of 400° C. or more in the presence of aninert gas before conducting the step (3).
 14. A process according to anyone of claims 1 to 13, which comprises a step (4) of contacting the gasobtained through the step (2-1), the step (2-2), the steps (2-1) and(2-3), the steps (2-1) and (3), the steps (2-2) and (3), or the steps(2-1), (2-3) and (3) with a gas separation membrane and/or a molecularsieving carbon.
 15. A process according to claim 14, wherein the gasseparation membrane is an SiO₂—ZrO₂ ceramic membrane and/or apoly(4-methylpentene-1) heterogeneity membrane.
 16. A process accordingto claim 14, wherein the molecular sieving carbon has a pore size of 5 Åor less.
 17. A high-purity tetrafluorosilane having ahexafluorodisiloxane content of 1 vol ppm or less, which is obtained bythe process as set forth in any one of claims 1 to
 16. 18 A high-puritytetrafluorosilane according to claim 17, wherein thehexafluorodisiloxane content is 0.1 vol ppm or less.
 19. A method foranalyzing impurities in a high-purity tetrafluorosilane, comprisingbringing tetrafluorosilane containing H₂ gas, O₂ gas, N₂ gas, CO gas,CH₄ gas and/or CO₂ gas, as impurities, into contact with an adsorbent toseparate said impurities from tetrafluorosilane, and introducing saidimpurities together with a carrier gas into a gas chromatograph toanalyze said impurities.
 20. A method according to claim 19, wherein theadsorbent is an activated carbon, a petroleum pitch spherical activatedcarbon and/or a molecular sieving carbon having a pore size of 6 Å ormore.
 21. A method for analyzing impurities in a high-puritytetrafluorosilane, comprising introducing tetrafluorosilane containinghexafluorodisiloxane as an impurity into a cell with the material ofwindow being composed of a metal halide, and analyzing thehexafluorodisiloxane and/or hydrogen fluoride by infrared spectrometry.22. A process according to any one of claims 1 to 16, wherein a methodas set forth in any one of claims 19 to 21 is used for the processcontrol.
 23. A gas for the production of an optical fiber, comprising atetrafluorosilane gas obtained by a process as set forth in any one ofclaims 1 to
 16. 24. A gas for the production of a semiconductor,comprising a tetrafluorosilane gas obtained by a process as set forth inany one of claims 1 to
 16. 25. A gas for the production of a solar cell,comprising a tetrafluorosilane gas obtained by a process as set forth inany one of claims 1 to 16.